@Anonymous User
Welcome to the forum.
You can have up to 4 different devices registered to your account, they all share the passes on your account and you can watch on up to 2 devices at the same time.
So you are fine to buy a new box and add it to your account, you won't have to buy extra passes.
NOW are looking for users to join their Customer Feedback Group and help improve the service. If you're interested read more here and sign up.